The present invention relates to a magnetostrictive motor of the inch worm type having performance characteristics that make it a capable of the precision control required for actuators.
Actuators are distinguished from ordinary drive motors by their controllability. Motors produce stable operation and convert energy efficiently. However, actuators do even more. They must provide superior control performance, high transient response, minimum delay time, high acceleration and damping, bidirectional symmetrical operation and wide tolerance for overload. Considering that modern control elements and sensors have progressed to a high level of accuracy and sophistication, actuators often prove to be the weak link in a system being able to achieve high precision, quick response and more power.
Magnetostrictive materials have enabled actuators to achieve larger force, fine positioning and backlash free applications. Since the early 1970's piezo electric materials (PZT) have been laminated to make active elements that can produce both high force and substantial displacement. In 1974 Burleigh Instruments Inc. designed an actuator in accordance with U.S. Pat. Nos. 3,902,084 and 3,902,085 which utilize three active piezo electric elements, two for clamping and one to generate forward motion. The two clamping elements are positioned on either end of an active, pushing or drive element. Motion is created by clamping a first end of the pusher, activating the pusher so that it expands. Clamping a second end, unclamping the first end and deactivating the pusher. To start the second cycle, the first end is clamped, the second end unclamped and the pusher again activated. A major problem with the latter design is that the elongation of the pusher is reduced in size when the built up strain is released, causing a slipping action. Moreover the significant hysteresis when the load shifts from push to pull reduces precision and stiffness.
Strains of 1,000 parts per million (ppm) can be achieved with PZT but the displacement decreases proportionately with the increase in force. Thus, PZT is suited only for relatively light loads, especially if large displacements are required. The advantage of PZT is its simplicity and compact structure. The disadvantages are the requirement for high voltage drivers, its low energy density and its vulnerability to de-polarization after momentary thermal overloads.
In the 1980's a rare earth alloy in the form of a large crystalline rod known as terfenol was found to exhibit unusually large magnetostrictive strain or elongation. Such crystals are grown as axially elongated rods in the direction of its crystalline axis. When magnetic fields are applied along the axis of these rods they exhibit large amounts of axial elongation. Terfenol exhibit strains of 2,000 ppm under modest magnetic fields. It operates at low voltage and has an energy density of more than 20,000 Joules per cubic meter, or in excess of 20 times greater than that of PZT. Terfenol produces compressive strengths of 700 MPa and the elongation is many time greater than that of PZT at any level of output force. The use of terfenol in inch worm linear motors has been disclosed in a paper entitled "An Application of Giant Magnetostrictive Material to High Power Actuators" by Der. Tomohiko Akuta (Paper No. 18A0107 at the 10th International Workshop on Rare-Earth Magnets and Their Applications, Kyoto, Japan, 16-18 May, 1989, (Proceedings Book: The Society of Non-Traditional Technology, 1-2-8, Toranomon, Minato-ku, Tokyo 105 Japan)) Akuta uses terfenol in place of PZT in a conventional inch worm motor design. Akuta also discloses a shear free clamp which allows conventional terfenol to be used in the clamps notwithstanding its low tensile strength. However, the slipping action inherent in the conventional inch worm motor design is still present in the Akuta design.
Accordingly, it is an object of the present invention to provide an inch worm motor which exhibits reduced slippage. It is a further object of the invention to provide an inch worm type of motor having an improved efficiency.